Living in southern Africa at the same time as Fabrosaurus, Heterodontosaurus was another of the early bird-hipped (ornithischian) dinosaurs. Like Fabrosaurus, it was small and fast-moving and lived on a diet of plants. In many ways, however, Heterodontosaurus was a more advanced animal than Fabrosaurus.

One for thing, it had more complex teeth. Heterodontosaurus gets its name from the three different types of teeth that lined its jaw. Those at the front of the mouth were small chopping teeth, while those at the back were larger, fatter, and more thickly enameled. These were suitable for grinding plant matter. Plant-eating is a challenging lifestyle - vegetable matter is often very tough and poor in nutrients. With these two types of teeth, Heterodontosaurus was able to eat plants more efficiently than any other kind of herbivore that had lived up to that time. It also had fleshy cheeks which helped the animal to keep extra food in its mouth while it was chewing.

Between the front and back teeth, Heterodontosaurus had two pairs of large tusks, similar to the canines of a carnivore. While these may have enabled Heterodontosaurus to eat meat as well as plants, it seems more likely that it used them for digging up roots or for displaying to rivals or to prospective mates. In some species of modern deer, only the males have tusks, but we do not know if Heterodontosaurus's tusks were present in only one or in both of the sexes.

The "thumb" of Heterodontosaurus's hand may have been opposable, and so it may have been used to grasp vegetation. The front legs were relatively long. Heterodontosaurus may have stood on all fours while grazing on ground plants, but if it needed to move quickly, it would have run on its powerful back legs.

Heterodontosaurus was the earliest of the ornithopod dinosaur. Many of the features that were important to this group's later success - batteries of grinding teeth, fleshy cheeks, the ability to run on the back legs - can be seen in ancestral form in this little dinosaur.